solving issues related to JENKINS-10628 / JENKINS-16116:

 - doCommitNotification considers GlobalNodeProperties environment variables before checks of svnUrl
 - repositoryLocationsNoLongerExist check within scmPolling considers GlobalNodeProperties environment variables before checks of svnUrl
 - doCommitNotification does not cut the first 4 characters of SVNUrl in order to Cut the "U " from the "U /your/changed/files" string instead it cuts the first character and trims the string. this was needed for compatibility to either post-commit hook or svn-version (didnt analyze this that precisely)